Dr. Sravani Kota
Final year P.G.
Atom
Atomic structure
Nucleus consists of Protons+ Neutrons
Ionization
Process of forming an ion
pair
Properties of X rays
•X rays are forms of electromagnetic radiation
•Short wavelength hence greater penetrating power
•Travel at speed of light 3X10 -8
•Travel in straight lines and have wave form
•X rays are made up of small packets of energy called photons or
quanta
•Follow inverse law
•Can penetrate opaque objects
•Can ionize gases
•Can stimulate or destroy living tissues
•Cannot be focused using a lens
X ray machine
X ray tube positioned within the tube head
Glass tube- Coolidge tube- 1913
Cathode, anode, power supply
Evacuated glass envelope to prevent collision
Cathode:
Filament- source of electrons- tungsten filament- 2mm, 1 cm
Focusing cup- Molybdenum- negatively charged helps in focusing
Filament mount on wires- connected to power supply
Anode:
Tungsten target embedded on a copper stem
Purpose- to convert kinetic energy of colliding electrons to X ray
photons
Focal spot- rectangular area on the anode
Sharpness of image increases when size of anode decreases
Angulated 20 degree- effective focal spot 1X1 actual- 1X3 mm
Why tungsten as anode?
High atomic number
High melting point
High thermal conductivity
Low vapor pressure
Factors Controlling the X-Ray Beam
Electrical factors
• Exposure time
• Tube current
• Tube voltage
Physical factors
• Filtration
• Collimation
• Inverse square law
EXPOSURE TIME
Exposure time is doubled, the number of photons generated at all
energies in the x-ray emission spectrum is doubled, but the range of
photon energies is unchanged.
TUBE CURRENT
As the mA setting is increased, more power is applied to the filament,
which heats up and releases more electrons that collide with the target to
produce radiation.
TUBE VOLTAGE
Increasing the kVp increases the potential difference between the cathode
and the anode, thus increasing the energy of each electron when it strikes
the target.
FILTRATION
x-ray beam consists of a spectrum of x-ray photons of different
energies
Photons that are of such low energy that they cannot reach the
receptor contribute to patient exposure (risk) but do not offer any
benefit.
To reduce patient dose, such low-energy photons should be removed
from the beam. This can be accomplished, in part, by placing an
aluminum filter in the path of the beam.
An aluminum filter preferentially removes many of the lower-energy
photons with lesser effect on the higher-energy photons that are able to
contribute to making an image.
Inherent filtration consists of the materials that x-ray photons
encounter as they travel from the focal spot on the target to form the
usable beam outside the tube enclosure- glass wall of the x-ray
tube, the insulating oil that surrounds many dental tubes
Total filtration is the sum of the inherent filtration plus any added
external filtration
COLLIMATION
A collimator is a metallic barrier with an aperture in the middle used
to reduce the size of the x-ray beam and thereby the volume of
irradiated tissue.
Round and rectangular collimators are most frequently used in
dentistry.
A round collimator is a thick plate of radiopaque material (usually
lead) with a circular opening centered over the port in the x-ray
head through which the x-ray beam emerges.
Rectangular collimators further limit the size of the beam to just
larger than the x-ray film, thereby further reducing patient
exposure.
INVERSE SQUARE LAW
For a given beam the intensity is inversely proportional to the square of the
distance from the source.
To summarize..